I have had a problem with the knob and cable that contols the switch that determines the direction of air flow for my a.c. in my truck.It was fixed once by the dealer,but broke again a day later.Why is it breaking,and how do I correct the problem.I have 2000 miles left on warranty,so I am worried it will break again and I will be stuck with a broken knob.It will turn but will not go into first position.

Go on the passenger side and look underneath where the passengers left foot would rest.
See if there's a cable that could have been kicked out of it's securing clip.
Move the select position knob in all positions to see if the cable stays still, and the blend door moves.
Do this again on the drivers side. Look straight up from the gas pedal. See if everything moves freely with the same knob.
If everything looks in place, then the control head will be next. Possible gears stripping etc.
This is not a common problem. I don't see much trouble with the Tundra's.